Urea-SCR (Selective Catalytic Reduction) systems are a known technology for decreasing NOx contained in the emissions from an internal combustion engine. Often, a selective reduction catalyst is placed in an engine exhaust passage to reduce NOx in the exhaust gases. A urea aqueous solution injection nozzle is placed upstream of the selective reduction catalyst for injecting urea (or urea reductants) into the exhaust stream. The injected urea is thermally decomposed or hydrolyzed into ammonia by heat from exhaust gas. Then, the ammonia is adsorbed into the selective reduction catalyst where it is used to reduce NOx to produce nitrogen (N2) and water (H2O). Thus, a denitrating reaction reduces NOx in exhaust gas.
In this regard, if the injected aqueous urea is not dispersed in the exhaust passage sufficiently, the ammonia generated from the urea will be adsorbed into the selective reduction catalyst disproportionately. As a result, the urea may produce more ammonia than desired in certain SCR regions while conversion of NOx may be reduced in other SCR regions.
One system for addressing this issue is described in JP2003-232218. In this reference, a mixing element for facilitating mixing of a urea aqueous solution injected from an injection nozzle with exhaust gas is provided in an exhaust passage located between the urea injection nozzle and a selective reduction catalyst. This mixing element comprises a division plate on which an opening is formed having a smaller cross-sectional area than a cross-sectional area of the plate. When exhaust gases contact the division plate and pass through the opening, the exhaust flow is disturbed. This causes the injected aqueous urea solution to mix well within the exhaust gases and to be more uniformly distributed to the SCR.
However, the inventors herein have discovered that some chemical compound crystals may attach to a surface of the mixing element. These crystals are thought to be derived from the injected urea. Specifically, thermal decomposition of urea is thought to form crystals of cyanuric acid made up of three isocyanic acid molecules. If such crystals continue to be deposited on the mixing element, the opening of the division plate becomes narrower, as it is blocked by the crystals and thereby increases exhaust gas-flow resistance. This can decrease engine torque performance and cause a decrease in engine output.
This detailed description addresses exhaust gas flow restriction in the presence of urea injection. In particular, the detailed description provides for an improved exhaust-gas treatment system that uses a urea-SCR.
One aspect of the present description includes a system for controlling performance of an internal combustion engine, the system comprising: an internal combustion engine having an exhaust system for processing gases exhausted from the internal combustion engine; the exhaust system comprised of at least one mixing device, at least one emission control device, and a delivery apparatus for delivering an agent affecting operation of the emission control device into the exhaust system, the at least one mixing device located downstream of the internal combustion engine and upstream of the at least one emission control device, and the delivery apparatus located upstream of the emission control device; and a controller to adjust at least one engine parameter in relation to an operating condition of the mixing device.
This system overcomes at least some of the disadvantages of the above reference.
By increasing the engine exhaust gas temperatures, gas-flow resistance in the exhaust passage related to the injected urea can be reduced. Consequently, engine torque and emissions can be substantially retained even if deposits occasionally form within an exhaust system. In one embodiment, a mixing element is placed in the exhaust system to improve mixing of the aqueous urea and the exhaust gases. Deposits attaching to the mixing device can be reduced by increasing engine exhaust gas temperatures. As a result, formation of deposits around the mixing device is reduced and the available engine torque is substantially maintained.
In a second aspect of the present description, a particulate trap is provided upstream of an exhaust gas mixing device and a selective reduction catalyst. Urea is introduced to the exhaust system upstream of the mixing device. A controller operates to raise engine exhaust gas temperatures when an exhaust gas flow is being restricted by either the particulate filter or the mixing device. This system also overcomes at least some of the disadvantages of the above reference.
In a third aspect of the present description, a method is provided for controlling operating conditions of an exhaust gas after treatment system that is coupled to an internal combustion engine, and includes an emission control device and a delivery apparatus for delivering an agent affecting operation of the emission control device into the exhaust gas after treatment system, the method comprising: purposefully increasing the exhaust gas temperature of the internal combustion engine when the pressure difference across an exhaust gas mixing device located within the exhaust gas after treatment system exceeds an amount. It will be appreciated that the purposeful increase may be achieved by programmatically controlling the internal combustion engine to increase the exhaust gas temperature.
The present description provides several advantages. Specifically, the system and method help to maintain engine torque if urea deposits form in an exhaust system. Further, the system and method promote mixing of aqueous urea and exhaust gases by reducing flow restrictions that may be caused by urea deposits. Further still, the method allows for particulate filter regeneration and urea deposit reduction at different times and frequencies. Consequently, less fuel may be necessary to operate the engine.
The above advantages and other advantages and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.